A fuel cell is an electrochemical energy conversion device and it produces electricity from various external quantities of fuel (anode side) and oxidant (cathode side). Generally these react in the presence of an electrolyte wherein the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Typically, fuel cells can operate virtually continuously as long as the necessary flows of reactants are maintained.
Many combinations of fuel and oxidants are possible. For example, a hydrogen cell uses hydrogen as fuel and oxygen as the oxidant. Other fuel cells include hydrocarbons and alcohols. Due to its abundance, coal has been proposed as a fuel cell reactant as well. This abundance coupled with ever-increasing populations around the world, there is a compelling and important need to find more efficient and responsible ways to use coal, such as with these fuel cells.
Attempts to use carbon in fuel cells has been tried. For example, one of the earlier attempts to directly consume coal in a fuel cell was made in 1966 when a carbon rod was used as the anode and platinum as the oxygen electrode in a fuel cell that employed molten potassium nitrate as the electrolyte. When oxygen was supplied to the platinum electrode, a current was observed in the external circuit. Nevertheless, these results were not encouraging because of the direct chemical oxidation of carbon by the potassium nitrate electrolyte.
Another attempt used molten sodium hydroxide electrolyte contained in an iron pot which served as the air cathode and a carbon rod as the consumable anode. The cell is operated at about 500° C. with current densities of over 100 milliamps per square centimeter. This attempt was plagued by the oxidation of the hydrogen and not the carbon, along with the sodium carbonate by the reaction of the carbon with molten sodium hydroxide, thus producing an undesirable side reaction involving the electrolyte, and rendering it unstable in that environment.
In the last several decades, high temperature fuel cells employing either molten carbonate or solid oxide ceramic electrolytes have been reported. In these fuel cells, coal-derived fuels were employed as consumable gas fuels. Presently, the high temperature solid oxide fuel cells under development use hydrogen derived either from natural gas or from coal. In addition to these attempts, there have also been attempts with molten salt electrolyte-based direct carbon fuel cells, molten anode-based direct carbon fuel cells and the like. With all of these attempts, the coal that is employed generally contains inorganics, minerals, and contaminants that produce ash and contaminants to be deposited in the bottom of the fuel cell that impedes the efficiency of the fuel cell. Further, when they deposit in the fuel cell, effort must be routinely expended to clean the fuel cell for its efficient operation.
Moreover, the energy density of carbon fuel cells is generally higher than that found with hydrogen and oxygen fuel cells. Due to the chemistry involved, the energy density for hydrogen and oxygen fuel cells produces approximately two electrons in a single reaction, whereas fuel cells using carbon anodes produce four electrons in a single reaction. Thus, the energy density is approximately twice that of a hydrogen fuel cell with a carbon fuel cell.